The present invention relates to a method and apparatus for introducing a compressed gas into a pressure vessel in which a gas is compressed in initial and final compression stages which can be powered by a common source. More particularly, the present invention relates to such a method and apparatus in which the compressed gas is produced by compressing a higher pressure gas stored in a higher pressure storage bank which is charged in part by compressing a lower pressure gas in the final compression stage. Even more particularly, the present invention relates to such a method and apparatus in which the gas is hydrogen and the pressure vessel is a vehicle fuel tank.
There exists a variety of industrial requirements for introducing compressed gases into pressure vessels, for instance, industrial gases and fuels such as compressed natural gas and hydrogen. With respect to hydrogen, there has been an increased interest in utilizing hydrogen as a fuel for internal combustion engines and fuel cells such as found in automobiles and buses. In order to store sufficient amounts of hydrogen to make the utilization of hydrogen practical for such purposes, the hydrogen must be stored in the vehicle fuel tank at high pressure, typically in excess of 5000 psig.
One important aspect in making the use of hydrogen practical is to provide equipment to conduct a filling operation that closely matches consumer expectations with respect to conventional fossil fuel filling. This expectation involves filling vehicle fuel tanks with hydrogen in between about 3 and about 5 minutes. Such rapid hydrogen filling can be accomplished with the use of high pressure cascaded storage tanks that sequentially dispense the hydrogen at high pressure. In cascade filling, when a consumer arrives at the filling station and connects a transfer hose to his or her vehicle, the fuel tank is connected to one of the cascaded storage tanks. Hydrogen flows out of the storage tank until pressure between the vehicle storage tank and the hydrogen storage tank equalizes. Hydrogen is then dispensed from another storage tank in the cascade to complete the filling of the tank. When one tank is dispensing, another tank can be filled or topped off with hydrogen.
The problem with cascade dispensing, as described above, is that the compressor that is used in connection with the filling of the cascaded tanks cannot be efficiently designed in that it has to be sized to compress the hydrogen from a low pressure to a high pressure to completely fill a tank and also to compress hydrogen from a medium pressure to a high pressure after a tank has undergone pressure equalization with a vehicle fuel tank. Hence, the compressor is oversized with respect to operations in a range from medium pressure to high pressure. Moreover, if the hydrogen is to be made on site, by for instance electrolysis, there are periods of time in which such equipment is not efficiently utilized in that the equipment must be turned down or off between filling periods.
Another large disadvantage with cascade dispensing is that direct tank to tank pressure transfer results in a lower pressure level in the vehicle as compared to the highest pressure storage bank. Assuming the banks are not re-pressurized between vehicle fills, any subsequent vehicle final pressure will be lower than the previous vehicle. This pressure decrease can be minimized by with the use of large storage volumes (higher capital and larger footprint penalty). As an option, the product compressor can be used to top off the vehicle tanks once the pressure transfers from the cascade banks are completed, however, this top off fill could take a considerable amount of time relative to consumer xe2x80x9cfast fillxe2x80x9d expectations.
The problem of dispensing fuels at high pressure has been encountered in connection with compressed natural gases. For instance in U.S. Pat. No. 5,351,726, a compressed natural gas refueling system is disclosed in which a single stage hydraulic compressor is provided to operate at suction pressures ranging from about 330 psig up to about 3600 psig and at discharge pressure ranging from 330 psig to 4500 psig. Compressed natural gas is supplied to an intermediate pressure storage tank at an intermediate pressure that is above the inlet source pressure for the natural gas. When vehicles are to be refueled, the vehicle is first refueled directly from the intermediate pressure storage tank. After equalization of pressure, the compressor compresses gas from the intermediate storage tank to the vehicle fuel tank to complete the refueling operation. Since the compressor does not have to function to compress natural gas from the supply pressure to the vehicle fill pressure at one time, the compressor itself can be a more compact energy efficient unit than a compressor size that would otherwise be required for such application.
In hydrogen filling applications, the hydrogen fill pressures are higher than those of the prior art involved in compressed natural gas. In addition, as with any vehicle filling station, footprint size becomes paramount. In addition, while there exists many sources of natural gas, hydrogen must be delivered and stored on site or at least made on site. In order to efficiently utilize systems that involve the manufacture of hydrogen on site, it is more efficient to allow for a continual utilization of the generation facilities. For instance, a facility that is designed to supply hydrogen on demand must be sized larger than a facility that continually supplies a nominal amount of hydrogen.
As will be discussed, the present invention provides a method of filling a vehicle fuel tank with hydrogen in which hydrogen sources can be continually used and that allows compact compression systems to be fabricated that are capable of compressing the hydrogen to the high vehicle fill pressures.
The present invention provides a method of introducing a compressed gas into a pressure vessel. In accordance with the method, a feed stream of a gas is compressed in an initial compression stage to form a lower pressure gas stream. During introduction of the compressed gas into the pressure vessel, a lower pressure storage bank is charged with the lower pressure gas stream, thereby to store lower pressure gas. Additionally, a compressed gas stream and therefore, the compressed gas, is introduced into a pressure vessel. The compressed gas stream is formed by compressing a stream of higher pressure gas in a final compression stage. The higher pressure gas is stored in a higher pressure storage bank.
During periods between the introduction of the compressed gas into the pressure vessel, the higher pressure storage bank can be charged with the compressed gas stream. The compressed gas is formed by compressing a combined stream in the final compression stage. The combined gas stream is composed of the lower pressure gas stream and a stream of stored lower pressure gas from the lower pressure storage bank.
The present invention advantageously allows the feed stream to be continually compressed and therefore continuously utilized. As will be discussed, the present invention has particular application to hydrogen fueling operations. As can be appreciated, since the source of hydrogen, for instance, an electrolysis unit or a steam methane reformer, can be made to continually produce said feed stream to be compressed, the source itself can be more fully utilized than prior art systems in which hydrogen is produced, stored and then dispensed. This allows the hydrogen source to be operated at a lower energy input, for instance, electricity in the case of electrolysis unit, or the source unit to be of a lessor scale than otherwise would have been required had the source unit been designed for intermittent operation. It is to be noted that the present invention also contemplates discontinuous operation.
The charging of a higher pressure storage bank can further comprise initially compressing the combined gas stream in the final compression stage and then completing the charging of the higher pressure storage bank by compressing a further combined stream in the final compression stage. The final combined stream can be composed of the lower pressure gas stream and part of the compressed gas stream. The remaining part of the compressed gas stream is then introduced in the higher pressure storage bank.
Alternatively, the higher pressure storage bank can be charged by again initially compressing the combined gas stream in the final compression stage and then completing the charging of the higher pressure storage bank by solely compressing the lower pressure gas stream within the final compression stage. In such embodiment, the final compression stage is a variable speed compressor and the speed of such variable speed compressor is reduced when the higher pressure storage bank is charged solely by compressing the lower pressure gas stream.
Since, during charging of the higher pressure storage bank, the lower pressure storage bank will become depleted, the available pressure of the gas stored within such storage bank will also be reduced. This will in turn reduce pressure at the suction side of the compressor. In order to maintain the suction pressure constant, either recycle or variable speed can be used as described above.
In another aspect, the initial and final compression stages can be provided with moving compression elements in which motion is imparted to the moving compression elements from a common source. In this regard the initial and final compression stages can be formed by first and second hydraulically powered reciprocating compressors, respectively. The feed stream is compressed in first and second cylinders of the first hydraulically powered reciprocating compressor. The first and second cylinders are operated at successively higher pressures. The combined stream in the stream of higher pressure gas are each alternately compressed in third and fourth cylinders of the second hydraulically powered reciprocating compressor. Hydraulic fluid is pumped by a common pump and used to impart motion to the moving compression elements formed by pistons within the first, second, third and fourth cylinders.
As discussed above, the pressure vessel can be a vehicle fuel tank and the gas can be hydrogen. In such case, the feed stream is at a source pressure of between about 70 psig and about 200 psig. The lower pressure storage bank is charged to a lower pressure between about 850 psig and about 1500 psig. The higher pressure storage bank is charged to a higher pressure of between about 850 psig and about 7000 psig.
It is to be noted that by imparting motion to moving compression elements from a common source, preferably hydraulically, the first and second compression stages can be mounted on a common skid to thereby decrease the footprint of the apparatus. In this regard, in another aspect of the present invention a method is provided for introducing a compressed gas into a pressure vessel. In accordance with such method, during periods between the introduction of a compressed gas into the pressure vessel, a feed stream is compressed in an initial compression stage to form a lower pressure gas stream. The lower pressure gas stream is compressed in a final compression stage to form a compressed gas stream. A storage bank is charged with the compressed gas stream to store higher pressure gas. During the introduction of the compressed gas into the pressure vessel, a compressed gas stream is introduced into the pressure vessel and the compressed gas stream is formed by compressing a stream of the higher pressure gas in the final compression stage. Motion is imparted to moving compression elements contained within the initial and final compression stages that act to compress the feed stream, the lower pressure gas stream and the stream of higher pressure gas, all from a common source.
Again, the initial and final compression stages can be formed by first and second hydraulically powered reciprocating compressors, respectively. The feed stream is compressed in first and second cylinders of the first hydraulically powered reciprocating compressor. The first and second cylinders are operated at successively higher pressures. The lower pressure gas stream and the stream of the higher pressure gas are each alternately compressed in third and fourth cylinders of the second hydraulically powered reciprocating compressor. Hydraulic fluid is pumped from a common pump to impart motion of the moving compression elements formed by pistons within the first, second, third and fourth cylinders. Again, the pressure vessel can be a vehicle fuel tank and the gas to be compressed can be hydrogen.
In another aspect of the present invention, an apparatus is provided for introducing a compressed gas into a pressure vessel. An initial compression stage is provided to compress a feed stream of a gas and thereby to form a lower pressure gas stream. A lower pressure storage bank is connected to the initial compression stage to allow the lower pressure storage bank to be charged with the lower pressure gas stream during introduction of the gas into the pressure vessel, thereby to store lower pressure gas therein. A final compression stage is selectively connected to a higher pressure storage bank to compress a stream of higher pressure gas stored in the higher pressure storage bank, thereby to produce a compressed gas stream composed of the compressed gas during the introduction of the compressed gas into the pressure vessel. Additionally, the final compression stage can be selectively connected to the lower pressure storage bank, the initial compression stage, and the higher pressure storage bank to compress a combined stream composed of the lower pressure gas stream produced by the initial compression stage and a stream of stored lower pressure gas from the lower pressure storage bank. This charges the higher pressure storage bank with the compressed gas stream during periods between the introduction of the compressed gas stream into the pressure vessel. A fill line is configured to be coupled to the pressure vessel and to be connected to the final compression stage to introduce the compressed gas stream and therefore, this compressed gas, into the pressure vessel.
A fill line is configured to be coupled to the pressure vessel and to be connected to the final compression stage to introduce the compressed gas stream and therefore, the compressed gas into the pressure vessel.
A recycle line can be connected to recycle part of the compressed gas stream in the final compression stage such that the charging of a higher pressure storage bank can be in part be effectuated by compressing a further combined stream in the final compression stage. The further combined stream can be composed of the lower pressure gas stream and part of the compressed gas stream. A remaining part of the compressed gas stream is introduced into the higher pressure storage bank. Alternatively, the final compression stage can be a variable speed compressor to allow a speed of the variable speed compressor to be reduced to in turn permit a speed of the final compression stage to be reduced. In such embodiment, the higher pressure storage bank is charged solely by compressing the lower pressure gas stream.
As stated above, the initial and final compression stages can have moving compression elements in which motion is imparted to the moving compression elements from a common source. In this regard, the initial and final compression stage can be first and second hydraulically powered reciprocating compressors. The first hydraulically powered reciprocating compressor can have first and second cylinders and the first and second cylinders can be connected in series to operate at successively higher pressures The second hydraulically powered reciprocating compressor can be provided with third and fourth cylinders connected for alternate compression. A common pump is provided to pump hydraulic fluid used in imparting motion to moving compression elements formed by pistons within the first, second, third and fourth cylinders.
This aspect of the present invention involving the common powering of compression stages, is applicable to embodiments that do not employ a lower pressure storage bank. As such, the present invention provides an apparatus for introducing a compressed gas into a pressure vessel. In accordance with this aspect of the invention, an initial compression stage is provided to compress a feed stream of a gas and thereby to form a lower pressure gas stream. A final compression stage is connected to a storage bank to compress a stream of higher compressed gas. Higher pressure gas stored in the higher pressure storage bank to produce a compressed gas stream during the introduction of the compressed gas into the pressure vessel. The initial and final compression stages can be formed by first and second hydraulically powered reciprocating compressors, respectively. A common pump can be provided to pump hydraulic fluids used to impart motion to moving compression elements formed by pistons within the first and second hydraulically powered reciprocating compressors. The storage bank in the initial compression stage can be selectively connected to the final compression stage during periods between the introduction of the compressed gas into the pressure vessel. This allows the lower pressure gas stream to be compressed within the final compression stage to produce the compressed gas stream and the compressed gas stream is then introduced into the storage bank to store the pressurized gas therewithin. The storage bank can also be selectively connected to the final compression stage during the filling of the pressure vessel to allow compression of a compressed gas stream of the compressed gas stored within the storage bank. A fill line is provided. The fill line is configured to be coupled to the pressure vessel and to be connected to the final compression stage to introduce the compressed gas stream and therefore the compressed gas into the pressure vessel. Again, the first hydraulically powered reciprocating compressor can have first and second cylinders to compress the feed stream. The first and second cylinders are connected in series to operate at successively higher pressures. The second hydraulically powered reciprocating compressor can have third and fourth cylinders to alternately compress the gas.